1. Field of the Invention
The present invention relates in general to data processing systems and, in particular, to a data processing system and method for producing an improved halftone screen. Still more particularly, the present invention relates to a data processing system and method for generating an improved halftone screen by shifting elements of a threshold matrix.
2. Description of the Related Art
In order to prepare a continuous tone image for printing utilizing a digital printer capable of printing only two tones, i.e. black or white, the image is first processed utilizing a halftone screen. The halftone screen converts the continuous gray tones of the original image into a bi-level image having only two tones.
The halftone screening process consists of using a screen pattern, and comparing the input image with the screen pattern. The screen is usually stored as a fairly small pattern that repeats itself or is replicated by programming. If the value of the image pixel is greater than the corresponding value of the screen cell, a output is printed. If the value of the image pixel is less than or equal to the screen cell value, no output is printed.
FIG. 1 illustrates a high level schematic diagram of an electronic screening process utilizing a threshold matrix in accordance with the prior art as described above. An original input image is input into a comparator 30. The other input into comparator 30 includes threshold values included within the screen threshold matrix. For each pixel in the output image, if the value of the corresponding pixel in the original image is greater than the corresponding threshold matrix value, the output image pixel (i) is printed as a black mark. If the value of the corresponding pixel in the original image is less than or equal to the corresponding threshold matrix value, the output image pixel (i) is not printed, i.e. it is left as a white space. In this manner, the entire original input image is halftoned utilizing the halftone screen by comparing each input value to its corresponding threshold value in the halftone screen matrix.
There are many cell shapes, screen frequencies, and screen angles used in halftoning. A straight line may be used as the cell shape where the line is oriented in the process direction of the printer in order to reduce the appearance of banding in the scan direction caused by the print engine.
This technique presents a variety of problems, however. For example, images containing light gray lines in the process direction may not print correctly. If the lines lie where the screen threshold values are low, they will be visible. If the lines are thin and lie along screen threshold values which are high, they may not be printed at all.
A data processing system and method are described for generating a halftone screened image from an input image. A first threshold matrix is generated which, when utilized to halftone a solid gray input image, generates a first halftoned output image which is a plurality of generally solid vertical lines. The first threshold matrix includes a plurality of matrix elements arranged in a plurality of rows and a plurality of columns. A plurality of groups of the matrix elements are shifted within the first threshold matrix to create a second threshold matrix which, when utilized to halftone the solid gray input image, generates a second halftoned output image which is a checkerboard type pattern of alternating black or white blocks. In one embodiment, groups of the plurality of the rows are shifted horizontally over by at least one adjacent column to generate the second matrix.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.